The invention relates to an arrangement for anti-lock control and for thrust control on a motor vehicle with all-wheel drive having a multiple-circuit fluid brake system controlled by a main cylinder with individual brakes for each of the wheels, with a main brake-fluid reservoir.
Devices of this type correspond to a level of in-house technical development attained by the application. However, German Published Unexamined Patent Application No. 3,006,137 shows a thrust-control device for vehicles with all-wheel drive, in which each driven vehicle wheel has assigned to it a pressure modulator which, by being connected under valve control to an operating-pressure source, can build up a brake system, in order thereby to prevent spinning of the vehicle wheels and keep their drive slip within a range of values linked to good thrust acceleration and to good driving stability.
The pressure modulators of the known thrust-control device have a drive piston which is designed as a differential piston and the larger piston step of which limits a drive-pressure space, into which the outlet pressure of the operating-pressure source can be fed. The differential piston also has a smaller piston step which limits an outlet-pressure space, from which projects the wheel-brake line leading to the wheel brake. As long as the thrust-control device is not activated, this outlet-pressure space is in communication, via a breather bore, with an outlet-pressure space of the brake unit, usually a tandem main cylinder. During normal braking, the outlet pressure generated in the tandem main cylinder as a result of pedal actuation is fed, via the outlet-pressure space of the modulator, into the wheel brake connected to the modulator. In this case, the outlet-pressure space of the pressure modulator acts as a line portion of the wheel-brake line.
When the thrust-control device is activated, so that the drive-pressure space of the pressure modulator is connected to the outlet of the operating-pressure source, then the differential piston of the pressure modulator is displaced in the direction of a pressure build-up in its outlet-pressure space. After the breather bore has been overrun, the modulator outlet-pressure space is shut off from the brake unit and the modulator itself acts as a brake unit which generates the brake pressure in a static hydraulic brake circuit.
A thrust-control device as described above can be combined perfectly well with an anti-lock device, it being expedient to insert between the pressure modulators and the wheel brakes connected to the brake-pressure regulating valves which can be controlled electrically by means of an electronic control device. These valves can be designed, for example, as three-way solenoid valves which can also be switched from a basic position, namely the pressure build-up position, into a pressure-holding position and a pressure-reduction position. In a design of the anti-lock device operating on the return principle, there would then have to be a return pump for each brake circuit of the brake system. Although the return pumps required for the individual brake circuits of a vehicle can have a common electrical drive, nevertheless such a combination of a thrust control and an anti-lock control would involve a very high outlay in technical terms, since electrically drivable pumps would have to be provided both for the anti-lock control device and the operating-pressure souce required for the thrust-control device.
In a further possible combination of the known thrust-control device with an anti-lock system operating on the bleed principle, although there would be no need for a return device, nevertheless, in this case, the brake system would have to be designed as a hydraulic auxiliary brake system. This system, even if the hydraulic auxiliary power source required for it were to be used for the thrust-control device, would involve a considerable technical outlay, since in addition to a brake valve of expensive design, it would also be necessary to provide measures to prevent "idle control" of the individual brake circuits. Further outlay would be necessary for this system, in order, in the event of a malfunction, for example a leak in one of the brake circuits, to make it possible to utilize the operating capacity of the brake circuit still intact.
How great this outlay is in technical terms can be inferred, for example, from German Published Unexamined Patent Application No. 3,421,776, which discloses a dual-circuit braking system for an all-wheel drive vehicle, which is equipped both with an anti-lock system operating in accordance with the release principle and also with a thrust control device.
It is also known (German Published Unexamined Patent Application No. 3,128,798) to construct an anti-lock system according to the principle of volume expansion and for this purpose to use, for example, pressure modulators which are individually associated with the wheel brakes capable of being subjected to control and which have a first control pressure base. A piston subjected to the output pressure of a tandem main cyclinder undergoes a displacement, by means of which braking pressure is built up statically in an outlet pressure space of the modulator. The wheel brake capable of being subjected to control is connected to this outlet pressure space as is a second control pressure space. The piston subjected - under valve control - to the outlet pressure of an auxiliary pressure force is again displaced back, against the pressure coupled into the first control pressure space, and thereby, by volume expansion of a outlet pressure space, braking pressure is diminished again in the associated wheel brake. This known arrangement, operating in accordance with the principle of volume expansion, for anti-lock control could indeed be combined with the initially described thrust control arrangement, likewise operating with pressure modulators, in such a manner that a respective pressure modulator for the thrust control and a pressure modulator for the anti-lock control are provided. However, such an arrangement would again be associated with a considerable outlay in technical terms.
An object of the invention is, therefore, to provide a device of the type mentioned in the introduction which can be produced at a markedly lower outlay in technical terms and which reliably meets the safety requirements placed on such a device.
According to the invention, this and other objects are achieved by means of an arrangement having a pressure modulator in at least one of the brake circuits for modulating fluid brake pressure applied to a corresponding brake to provide braking, anti-locking and thrust control of said brake. Connected to the pressure modulator is an operating-pressure supply means having a reservoir and a pressure outlet, for supplying and receving fluid to and from the pressure modulator. The arrangement also comprises valve means for controlling flow of the fluid between the presure modulator and the brake, and between the pressure modulator and the operating-pressure supply means. A valve control means controls the operation of the valve means.
A hydraulic separation is obtained by the pressure modulators of the brake-pressure control circuits assigned to the individual wheel brakes from the outlet circuits, to which the wheel brakes are connected. This has the advantage that each wheel has its own brake circuit, with the result that, in the event of a malfunction in one of these wheel-brake circuits, there is not reaction in the other wheel-brake circuits. If there is a failure of one of the wheel-brake circuits, very good braking deceleration can still be achieved by means of the further wheel-brake circuits which remain operational.
The separation of the wheel-brake circuits from the control unit also provides in an elegant way the precondition for ensuring that the brake-pressure control for the anti-lock control can take place according to the principle of an increase in volume, in other words, as a result of a retraction of the control piston of the particular pressure modulator. This in a simple way prevents "idle control" of the individual brake circuits and guarantees a high operating reliability of the brake system as a whole.
The design of the control piston and its arrangement in the pressure modulator, along with the drive pistons and the spring, allow an elegant control of the advancing and retracting movements of the modulator control pistons which are necessary for the anti-lock and the thrust control.
The advantage provided by designing the control piston with two piston flanges having an annular space space between them is that a leak in the region of the pressure modulator can be detected reliably from a "flopping" of the brake pedal, and consequently repair measures can be carried out in good time before a complete failure of the brake system can occur.
By designing the drive-pistons a differential pistons in contemplated embodiments, it is possible in a simple way to match the pressure modulators to the outlet-pressure level of an operating-pressure source which can be lower than the maximum utilizable brake pressure for which the brake system is designed. Then, for example, an auxilliary-pressure source, operating at a relatively low pressure level and present on the vehicle in any case, of a power steering system or levelling control can be used as an operating-pressure source.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.